Interference involving multiple reflections is known in the art. This phenomenon is described at “Fundamentals of Optics”, F. A. Jenkins and H. E. White, 4th edition, 1976, pages 286–314. The interference results from multiple reflections of light between two substrates of a thin layer (or film) of transparent material. In other words, a single beam of radiation that is directed towards a thin layer of transparent material that is followed by an object that has a refractive index different from the film causes multiple beams of radiations to be reflected from the thin layer. If the object is transparent or partially transparent the single beam of radiation may result in additional multiple transmitted beam of radiations. The multiple reflected beams of radiations undergo distinct optical paths and generate interference patterns.
This phenomenon may occur in bright field inspection systems for inspecting various objects such as semiconductor wafers. In many cases a thin layer of dielectric material coats the inspected layer of interest. A typical dielectric layer is made of SiO2, silicon nitride, photo resists and so on. Due to the difficulties of producing a uniform layer across the entire wafer during chip manufacturing process, small thickness changes may exist for different dies. These thickness variations are insignificant for the electrical performance of the IC's but may cause significant reflectivity changes in the course of bright field inspection. When wafer inspection includes a comparison of die-to-die images these reflectivity variations may greatly degrade the comparison outcome.
A typical optical inspection system has an objective lens that focuses a beam of radiation onto an inspected object. The beam of radiation can be viewed as a plurality of light rays that are directed towards the inspected object at multiple incidence angles. Objective lenses that are characterized by high numerical apertures can be viewed as being able to direct light rays from large range of incidence angles towards the inspected object.
The optical paths that the multiple rays of light pass until being detected depend upon the thickness of the dielectric layer and the incidence angle. As the thickness of the dielectric layer may vary in a manner that is usually unknown to the detection system, the detected signal may be responsive to the unknown thickness of the layer. The detected signal can vary substantially as a result of said unknown thickness thus causing detection errors.
A prior art optical inspection system may consist of a light source, a high NA objective lens, a detector and associated electronics and software. The prior art system illuminates an inspected object such as but not limited to a wafer, mask, reticle, and the like. The high NA objective lens is used to illuminate the inspected object as well as to collect the back-reflected light, which is then focused onto a detector.
An illuminated wafer can be viewed as being made of a silicon substrate with a thin layer of a dielectric coating on it, though the effective substrate material may be different at different stages of the manufacturing process. In order to calculate the effect of the wafer on the back reflected beam of radiation, the spherical converging beam of radiation that is emitted from the objective is represented by a sum of plane waves with the same wavelength but propagating in different directions. The reflectivity R as function of incident angle is given by the equations below:
                    R        =                                            R              ∥                        +                          R              ⊥                                2                                                  R          =                                                                      (                                      r                    12                    η                                    )                                2                            +                                                (                                      r                    23                    η                                    )                                2                            +                              2                ⁢                                  r                  12                  η                                ⁢                                  r                  23                  η                                ⁢                Cos                ⁢                                                                  ⁢                2                ⁢                β                                                    1              +                                                                    (                                          r                      12                      η                                        )                                    2                                ⁢                                                      (                                          r                      23                      η                                        )                                    2                                            +                              2                ⁢                                  r                  12                  η                                ⁢                                  r                  23                  η                                ⁢                Cos                ⁢                                                                  ⁢                2                ⁢                β                                                    ;                  η          =                      ∥            or            ⊥                                                                                      r                              j                ,                k                                      ∥                    =                                                                      n                  j                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  j                                            -                                                n                  k                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  k                                                                                                      n                  j                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  j                                            +                                                n                  k                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  k                                                                    ;                                                                    r                              j                ,                k                                      ⊥                    =                                                                      n                  k                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  j                                            -                                                n                  j                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  k                                                                                                      n                  k                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  j                                            +                                                n                  j                                ⁢                Cos                ⁢                                                                  ⁢                                  θ                  k                                                                    ;                                                                    n              k                        ⁢            Sin            ⁢                                                  ⁢                          θ              k                                =                                    n              j                        ⁢            Sin            ⁢                                                  ⁢                          θ              j                                      ;                              (                          j              ,              k                        )                    =                                    (                              1                ,                2                            )                        ⁢                                                  ⁢            or            ⁢                                                  ⁢                          (                              2                ,                3                            )                                                                        β          =                                                    2                ⁢                π                                            λ                0                                      ⁢                          n              2                        ⁢            h            ⁢                                                  ⁢            Cos            ⁢                                                  ⁢                          θ              2                                      ;            
Where θk is the refracted ray angle within the layer k (k=1,2,3 denoting air, dielectric or substrate, respectively) relative to the substrate normal, h is the dielectric layer thickness, nk is the respective refractive index of layer k, and λ0 is the wavelength of the incident light in air. The intensity at the detector is the sum of intensities of all incident rays.
It is known in the art that the reflected intensities for different rays depend upon the coating layer thickness h, and that this dependence has a periodical behavior, as illustrated by the curves of FIG. 1A. The period is inversely proportional to the thickness h of the thin dielectric layer. Each curve represents the dependence of the reflected intensity of rays on the angle of incidence, for a single thin layer thickness.